The first system of long distance communication by electrical means was, of course, telegraphy using a code of dots and dashes, that is, short and long pulses. The exact length of dot and dash in telegraphy is not significant so long as one is recognizable as being different from the other. Telegraphy is still widely used both in wired and wireless communication. Teletypewriter apparatus converts alphanumeric characters to code at the sending end and reconverts the code to alphanumeric characters at the receiving end. In early radio communication the transmitted frequency was interrupted to form dots and dashes but that form of communication has been largely supplanted by continuous wave communication in which an audible frequency modulates the carrier and the modulating frequency is keyed or interrupted for dots and dashes. The amplitude, frequency or phase of a carrier can be modulated, and the modulation may be in the form of pulses, as in radar, or by encoding the amplitudes of a sampled signal, as in pulse code modulation.